U.S. Pat. No. 8,022,802 B2 relates to a sensor for measuring electrical parameters in a high voltage environment and comprises isolation transformers in several embodiments, including one embodiment with a single main circuit board for a plurality of adjacently disposed windings which are coupled magnetically by magnetic core rings, and a further embodiment with a main circuit board, a secondary circuit board and two magnetic core rings which extend through openings of the main and secondary circuit boards. The primary winding and the secondary winding are arranged in and on the main circuit board, while the coupling winding for coupling the two magnetic core rings is arranged on the secondary circuit board. There is some intermediate space between the two circuit boards, and furthermore the coupling winding on the secondary circuit board is spaced from the respective edge of the openings in the magnetic core rings. In this way, relatively large ring core openings are required in the magnetic core rings.
In another transformer (DE 10 2005 041 131 A1), windings are wound around the ferromagnetic cores to form coils, and the windings of the coils are arranged on different ferromagnetic cores in order to maintain required isolation distances. The ferromagnetic cores are magnetically coupled to one another by an additional winding embedded in a circuit board. Because the winding has to be wound around the ferromagnetic cores, manufacturing of a transformer of this configuration is only possible at high costs.
From US 2011/0140824 A1 a transformer is known in which windings that have to be separated with respect to their potential are asymmetrically arranged on different circuit boards which are stacked and connected to form the transformer using a two-part ferromagnetic core.
US 2011/0095620 A1 discloses a planar transformer for miniaturized applications which has coil windings disposed on opposite sides of an insulating substrate. The device operates based on induction, without ferromagnetic cores.
EP 0 715 322 A1 discloses a planar type transformer comprising conductor tracks that are disposed in layers of a circuit board thus forming transformer windings. A ferromagnetic core surrounds the transformer windings, with outer annular legs and with a cylindrical inner leg.
DE 20 2009 002 383 U1 discloses a planar transformer comprising a multi-layered circuit board which ensures high dielectric strength among the layers of the circuit board between the primary and the secondary windings. The transformer can be driven floating with opposing signals. A signal to be transmitted in the positive direction of magnetic flux of a common primary winding or an individual primary winding directly generates a positive control signal in a first secondary winding in the same coupling direction. A signal in the negative direction of magnetic flux of a second or of the same winding directly generates a likewise positive control signal in a second secondary winding in a coupling direction opposite to that of the first secondary winding, or a negative control signal in the first secondary winding, and if no further signal is to be transmitted, the transformer is automatically or digitally controlled by circuit elements so as to be demagnetized by driving one or two windings in short circuit directly at the end of a previously transmitted signal.
DE 10 2009 037 340 A1 discloses a transformer in which annular cores with windings are coupled with each other by a short-circuit winding. The short-circuit winding is connected to respective contacts of a circuit board, for example by soldering.
The invention is based on the object to provide a planar transformer that is easily manufactured and that provides for electrical isolation or potential separation for two or more potential groups in a very small space.